The present invention pertains to a device and method for recycling photons in a light emitting diode. In light emitting diodes (LED) photons are created by the biasing of a P-N junction. Electrons flow from the N-doped material to the P-doped material. Once in the P-doped material, the electrons combine with holes to emit photons. These photons are emitted in every direction, with a very small percentage reaching the edges of the LEDs and escaping to contribute to the external efficiency of the device. Only weakly absorbed photons have a relatively high probability of escaping, and reduced absorption can be realized only if the photon energy is substantially less than the bandgap energy of the material through which the photon must pass.
In prior art, graded bandgap LEDs, such as that in Dawson "High-efficiency graded-band-gap Ga.sub.1-x Al.sub.x As light-emitting diodes" J. Appl. Phys., Vol. 48, No. 6, June 1977, attempt to increase the external efficiency. Dawson used liquid phased epitaxy to grow a P-N junction on a substrate. The P-N junction is formed during the growth of a single epitaxial layer. This arises from the fact that silicon is an amphoteric dopant in gallium aluminum arsenide (GaAlAs) grown from Ga rich solutions, substituting in greater numbers on Ga and Al sites (as donors) at high growth temperature or As sites (as acceptors) at low growth temperature. By cooling a Ga, Al, As, Si solution in contact with the GaAs substrate through the N to P transition temperature, a P-N junction is formed without removal of the solid from the growth system and without changing the growth liquid composition.
In LED's, photons escape the device through an exit surface referred to here as the window. In Dawson, the internal absorption of photons moving towards the window is reduced by the use of a graded bandgap. The graded bandgap arises because of a variation in the amount of Al incorporated during the growth of these thick epitaxial layers. Under these growth conditions, Al segregates strongly in favor of the solid, leading to a decrease in AlAs mole fraction as growth proceeds due to depletion of Al from the bulk of the liquid.
This graded bandgap means that the bandgap energy, as the photon approaches the window, is increasing in the material through which the photon is moving. The bandgap energy of the material, being higher than the energy of the photon, means that the internal absorption of the photon is reduced. Also, the photons not headed toward the window are reabsorbed deeper in the device away from the window due to the reduction of the bandgap energy in that direction. These reabsorbed photons will then generate electron hole pairs. The photogenerated then recombine with holes to emit more photons with the lower energy of the bandgap of the material at the greater depth. These photons have less energy than the previous photons; therefore, the photons of this group moving towards the window again have energy lower than the material bandgap through which they pass thus having less chance to be absorbed. This cycle continues depending on the thickness of the layer. While this device increases the external efficiency, it has some problems.
There are problems with graded bandgap LEDs. In order for the graded bandgap to be in the correct direction with the highest bandgap energy by the window, the substrate must be removed. Substrate removal is an expensive and time consuming process. Furthermore, since the substrate is removed, the grown layer must be thick enough to survive structurally. Also, while the photons are recycled, the energy reduction from one photon to the next can be quite substantial thereby reducing the number of cycles that can occur. This is due to the electron moving a substantial distance down the bandgap energy gradient before recombination with holes can occur.